Breast cancer risk is strongly determined by numerous genes, only a few of which have been identified. The female ACI rat exhibits a unique, genetically-conferred, susceptibility of to 172-estradiol (E2)-induced mammary cancer. Because estrogens have been inextricably implicated in the etiology of breast cancer, the ACI rat is rapidly gaining wide acceptance within the breast cancer research community as a physiologically relevant model. To define the genetic bases of susceptibility to E2-induced mammary cancer, we performed intercrosses between the susceptible ACI strain and the resistant Copenhagen (COP) or Brown Norway (BN) strains and revealed 9 quantitative trait loci (QTL) that significantly impact susceptibility to mammary cancer. Two of these QTL, Emca1 and Emca8, were mapped to an overlapping region of rat chromosome 5 (RNO5), and a third, Emca4, was mapped to RNO7. Data summarized herein strongly suggest that the regions of the human genome that are orthologous to these three Emca loci harbor as yet unidentified breast cancer susceptibility genes. The objectives of this research are to identify genes residing within Emca1, Emca4 and Emca8 that determine breast cancer risk and to define better the molecular mechanisms through which estrogens contribute to breast cancer development. Aim 1 is to identify genes residing within Emca8 that determine susceptibility to E2-induced mammary cancer. A substitution mapping approach employing congenic rat strains will be used to fine map Emca8. Development of each congenic strain will be directed by preliminary data that are indicative of genetic heterogeneity across Emca8. Expression of genes residing within a minimal effective congenic interval will be evaluated at the mRNA and protein levels. Nucleotide sequence across the minimal congenic interval will be determined for the ACI and BN strains to identify the genetic variant responsible for the observed difference in susceptibility. Aim 2 is to determine whether Emca1 and Emca8 harbor the same determinants of susceptibility to E2-induced mammary cancer. Genes identified in Aim 1 that mediate the actions of Emca8 on mammary cancer susceptibility will be evaluated in the COP strain to determine if the COP and BN rat strains share alleles for these genes. Aim 3 is to identify the genetic variant residing within Emca4 that determines susceptibility to E2-induced mammary cancer. This aim will be focused on the region of Emca4 that is orthologous to a recently mapped, but as yet unidentified, genetic determinant of breast cancer risk in humans. These proposed studies promise to greatly enhance our knowledge regarding how breast cancer risk is genetically determined, as well as to the mechanisms through which estrogens contribute to breast cancer development. Relevance to public health: This research utilizes a novel and physiologically relevant rat mammary cancer model to identify genes that determine mammary cancer susceptibility. Because of the numerous similarities between this model and breast cancer in humans, it is believe that the genes identified in this study will similarly impact breast cancer risk in humans. These studies will also reveal insight into the mechanisms through which estrogens contribute to breast cancer development.